The present invention relates to a portable visual display and, more particularly, to a head-mounted visual display having see-through function (hereinafter referred to as head-mounted see-through display), which can be retained on the observer's head or face.
Helmet- and goggle-type visual displays have heretofore been developed for the purpose of enabling the user to enjoy virtual reality or a wide-screen image by oneself. In particular, a visual display having see-through function has recently been developed to enable a real world image outside the user and an electronic image displayed on a two-dimensional display device to be superimposed on one another or to allow the user to change over these two images from one to the other by a switching operation.
Japanese Patent Application Laid-Open (KOKAI) No. 3-226198 (1991) discloses a method wherein a liquid crystal shutter is used as a means for changing over the electronic image and the outside image in a visual display of the type described above. However, Japanese Patent Application Laid-Open (KOKAI) No. 3-226198 makes no mention of a specific arrangement of the liquid crystal shutter, although it discloses an idea of using a liquid crystal shutter for the see-through function.
As another prior art, Japanese Patent Application Laid-Open (KOKAI) No. 3-189677 (1991) discloses a technique in which a single liquid crystal shutter is used in such a way that when an outside image is to be viewed, the shutter is made transparent, whereas, when an electronic image is to be viewed, the shutter is made opaque. However, no mention is made of a specific arrangement of the liquid crystal shutter. Accordingly, this is a first problem to be solved by the present invention.
Further, the present inventors noticed the fact that a liquid crystal shutter which employs a twisted nematic (TN) liquid crystal or a super-twisted nematic (STN) liquid crystal has viewing angle dependence. That is, when such a liquid crystal shutter is seen from a specific direction, it exhibits a high contrast (this direction is referred to as "viewing angle direction"; the term is also applied to projection that is made on the liquid crystal shutter in this direction). However, when the liquid crystal shutter is seen from other directions, the contrast is relatively low.
In the case, for example, of a positive type liquid crystal shutter (the term "positive type" denotes a type of liquid crystal shutter which becomes opaque when a voltage is applied thereto) of 6-hour viewing angle in which the viewing angle direction is the 6-hour direction, as shown in FIG. 11(a), the relationship between the viewing angle .THETA..sub.0 in the vertical direction (the 12- to 6-hour direction) and transmittance of the liquid crystal shutter when a voltage is applied thereto, that is, when the shutter is opaque, exhibits characteristics as shown in FIG. 11(b). The relationship between the viewing angle .THETA..sub.1 in the horizontal direction (the 3- to 9-hour direction) and the transmittance at that time is such as that shown in FIG. 12(b). As will be clear from FIGS. 11(b) and 12(b), the region where a favorable opaque condition is available is approximately symmetric in the horizontal direction, but it is asymmetric in the vertical direction.
Accordingly, a head-mounted display with see-through function that employs a positive type liquid crystal shutter involves the following problems: When an electronic image is viewed, in a region close to the center of the electronic image (i.e., a region in which the field angle is small) the viewing angles .THETA..sub.0 and .THETA..sub.1 with respect to the liquid crystal shutter are small and hence the opacity is high. Therefore, substantially no outside image can be seen through the liquid crystal shutter in the central region of the electronic image. However, the viewing angles .THETA..sub.0 and .THETA..sub.1 become larger as the distance from the central region increases toward the peripheral region (where the field angle is large), and it becomes difficult to obtain an adequate opacity. Consequently, the outside image is undesirably seen through the liquid crystal shutter, so that the electronic image becomes difficult to view.
When the field angle of the electronic image is further increased as in the case of wide vision, it becomes further difficult to obtain an adequate opacity in the peripheral region of the electronic image, and the outside image is undesirably seen through the liquid crystal shutter.
Even if the system is successfully arranged so that the electronic image can be put within a viewing angle range in which the opacity of the positive type liquid crystal shutter can be sufficiently obtained by reducing the field angle of the electronic image, such an arrangement imposes restrictions on the position and angle where the liquid crystal shutter is attached to the head-mounted display body, thus making it difficult to reduce the overall size of the head-mounted display. This is a second problem to be solved by the present invention.
In addition, if it is contrived to reduce the overall size of the head-mounted display, the position where the liquid crystal shutter is installed comes close to the eye. Therefore, the liquid crystal shutter itself can be reduced in size. However, when the electronic image alone is to be viewed, that is, when the liquid crystal shutter is made opaque, if the opacity is not sufficiently high, the outside image may be seen through superimposed on the electronic image. Further, a pinhole may be present in a liquid crystal shutter. If the diameter of the pinhole is large, the outside image may be seen through the pinhole, which obstructs the vision of the electronic image. This is a third problem to be solved by the present invention.